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In fact the relation that we uncovered in [16] between the Hopf algebra of Feynman<br />

graphs and the Hopf algebra of coordinates on the group of formal diffeomorphisms of<br />

the dimensionless coupling constants of the theory allows to prove the following result<br />

which for simplicity deals with the case of a single dimensionless coupling constant.<br />

Let the unrenormalized effective coupling constant g eff (ε) be viewed as a formal power<br />

series in g and let g eff (ε) = g eff+ (ε) (g eff− (ε)) −1 be its (opposite) Birkhoff decomposition<br />

in the group of formal diffeomorphisms. Then the loop g eff− (ε) is the bare coupling<br />

constant and g eff+ (0) is the renormalized effective coupling.<br />

This allows, using the relation between the Birkhoff decomposition and the classification<br />

of holomorphic bundles, to encode geometrically the operation of renormalization. It<br />

also signals a very clear analogy between the renormalization group as an ”ambiguity”<br />

group of physical theories and the missing Galois theory at Archimedian places alluded<br />

to above. We refer to [14] for a detailed account of this analogy.<br />

I<br />

References<br />

[1] M.F. Atiyah, Global theory of elliptic operators, Proc. Internat. Conf. on<br />

Functional Analysis and Related Topics (Tokyo, 1969) University of Tokyo<br />

press, Tokyo, (1970), 21-30.<br />

[2] P. Baum and A. <strong>Connes</strong>, Geometric K-theory for Lie groups and foliations.<br />

Preprint IHES (M/82/), 1982, to appear in l’Enseignement Mathematique,<br />

t.46 (2000), 1-35.<br />

[3] L.G. Brown, R.G. Douglas and P.A. Fillmore, Extensions of C ∗ -algebras and<br />

K-homology, Ann. of Math. 2, 105 (1977), 265-324.<br />

[4] A. Chamseddine and A. <strong>Connes</strong>: Universal formulas for noncommutative geometry<br />

actions, Phys. Rev. Letters 77 24 (1996), 4868-4871.<br />

[5] J. Chabert, S. Echterhoff, R. Nest, The <strong>Connes</strong>-Kasparov conjecture for almost<br />

connected groups, MathQA/0110130.<br />

[6] A. <strong>Connes</strong>, Une classification des facteurs de type III, Ann. Sci. Ecole Norm.<br />

Sup., 6, n. 4 (1973), 133-252.<br />

[7] A. <strong>Connes</strong>, Classification of injective factors, Ann. of Math., 104, n. 2 (1976),<br />

73-115.<br />

[8] <strong>Connes</strong>, A., C ∗ algèbres et géométrie differentielle. C.R. Acad. Sci. Paris,<br />

Ser. A-B 290 (1980).<br />

[9] A. <strong>Connes</strong>, Noncommutative differential geometry. Part I: The Chern character<br />

in K-homology, Preprint IHES (M/82/53), 1982; Part II: de Rham homology<br />

and noncommutative algebra, Preprint IHES (M/83/19), 1983. Noncommutative<br />

differential geometry, Inst. Hautes Etudes Sci. Publ. Math. 62<br />

(1985), 257-360.<br />

[10] <strong>Connes</strong>, A., : Cohomologie cyclique et foncteur Ext n . C.R. Acad. Sci. Paris,<br />

Ser.I Math 296 (1983).<br />

22

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